symbology - Applying catgorized symbol to each feature using PyQGIS

I want to apply unique symbol to each feature. i have used following code for doing that

from qgis.core import *
from PyQt4.QtGui import *
# supply path to where is your qgis installed
QgsApplication.setPrefixPath("/path/to/qgis/installation", True)
# load providers

QgsApplication.initQgis()

# Get the active layer (must be a vector layer)
layer = qgis.utils.iface.activeLayer();
iter = layer.getFeatures();
for feature in iter:
    geom = feature.geometry()
    print "Feature ID %d: " % feature.id()
    qgis.utils.iface.mapCanvas().setSelectionColor( QColor("yellow") );
    layer.setSelectedFeatures([feature.id()])

    qgis.utils.iface.mapCanvas().zoomToSelected( layer )
    qgis.utils.iface.mapCanvas().refresh()
    selected_features = layer.selectedFeatures()
    for i in selected_features:
       attr =i.attributes()
    chaltano= str(attr[layer.fieldNameIndex('test')])
    renderer = QgsCategorizedSymbolRendererV2("test")
    layer.setRendererV2(renderer)
    symbol = QgsSymbolV2.defaultSymbol(layer.geometryType())
    symbol.setColor(QColor("red"))

    cat = QgsRendererCategoryV2(feature.id(), symbol,str(feature.id()))
    renderer.addCategory(cat)

Symbol is applying to only last feature. Here, "test" is attribute.

Answer

To draw a layer with a categorized renderer you first should create appropriate categories. A category needs value, symbol, and label. If you know the values at design time you may define these 3 parameters for each category. For an example see QGIS Python Programming Cookbook.

After defining categories you create a QgsCategorizedSymbolRendererV2() with these categories, and assign this renderer to the layer.

My code below creates a category for each unique value in a given field, with a random color. I override the default symbol to allow for some flexibility in styling.

from random import randrange


# Get the active layer (must be a vector layer)
layer = qgis.utils.iface.activeLayer()

# get unique values 
fni = layer.fieldNameIndex('test')
unique_values = layer.dataProvider().uniqueValues(fni)

# define categories
categories = []
for unique_value in unique_values:

    # initialize the default symbol for this geometry type
    symbol = QgsSymbolV2.defaultSymbol(layer.geometryType())

    # configure a symbol layer
    layer_style = {}
    layer_style['color'] = '%d, %d, %d' % (randrange(0,256), randrange(0,256), randrange(0,256))
    layer_style['outline'] = '#000000'
    symbol_layer = QgsSimpleFillSymbolLayerV2.create(layer_style)

    # replace default symbol layer with the configured one

    if symbol_layer is not None:
        symbol.changeSymbolLayer(0, symbol_layer)

    # create renderer object
    category = QgsRendererCategoryV2(unique_value, symbol, str(unique_value))
    # entry for the list of category items
    categories.append(category)

# create renderer object
renderer = QgsCategorizedSymbolRendererV2('test', categories)


# assign the created renderer to the layer
if renderer is not None:
    layer.setRendererV2(renderer)

layer.triggerRepaint()